1. Technical Field
The invention relates to a process for recovering hydrocarbons from a subterranean formation, and more particularly to a hydrocarbon recovery process using a permeability-reducing material.
2. Background Information
Selective placement of a treatment material in a particular location of a subterranean hydrocarbon-bearing formation is oftentimes critical to the success of a treatment process designed to facilitate the recovery of hydrocarbons from the formation. In contrast, non-specific placement of the treatment material in the formation can negate the treatment process, diminishing rather than facilitating hydrocarbon recovery from the formation.
The present invention is one such treatment process that requires selective placement of the treatment material in the formation. The invention is directed in particular to selective placement of the treatment material in a subterranean hydrocarbon-bearing formation to inhibit vertical flow of fluids between a well bore and an aquifer underlying a hydrocarbon production or fluid injection interval of the formation, wherein the interval is in fluid communication with both the aquifer and the well bore.
When hydrocarbons are radially produced from the formation through the production interval into the well bore, water from the underlying aquifer tends to migrate vertically through water-permeable flow paths in the production interval to the near well bore region of the production interval. This phenomenon, termed "water coning," has two primarily negative consequences. The first is that water entering the production interval can block or significantly diminish the radial flow of hydrocarbons from the outlying formation into the well bore. The second is that water production can compete with and diminish hydrocarbon production. As a result, the ratio of water to hydrocarbons produced from the well bore can become unacceptably high.
The inverse problem to water coning exists when hydrocarbon recovery fluids such as hydrocarbon displacement fluids are injected into the injection interval via the well bore to displace hydrocarbons toward an adjoining hydrocarbon production well bore. If fluid communication exists between the injection interval and the underlying aquifer, particularly in the near well bore region, the displacement fluid has a propensity to be undesirably diverted away from the radially outlying hydrocarbon-bearing formation into the underlying aquifer where the displacement fluid is dissipated without effectively contacting any hydrocarbons.
The desirability of selectively placing a treatment material in a lower portion of the production or injection interval of a formation to inhibit vertical flow of fluids between the well bore and underlying aquifer has been demonstrated in U.S. Pat. No. 5,067,564 to Sydansk. In accordance with the teaching of Sydansk, a gelation solution and a gas are sequentially injected into the well bore being treated. The gas overdisplaces the gelation solution from the upper portion of the interval while permitting the gelation solution to remain in the lower portion of the interval where it sets up as a permeability-reducing gel.
The process of Sydansk is not entirely satisfactory, however, insofar as sequential injection of the gelation solution and gas into the formation can be relatively time consuming and correspondingly costly as the result of extended production or injection well downtime. Furthermore, some residual gelation solution may remain in the upper portion of the interval even after the upper portion is swept with an overdisplacement gas, thereby undesirably diminishing the subsequent permeability of the upper portion to hydrocarbons or recovery fluids.
U.S. Pat. No. 4,387,770 to Hill discloses a process for selective placement of a treatment fluid in a formation. A pair of fluids having different densities are initially placed in the well bore and permitted to gravity segregate therein. A treatment fluid having an intermediate density is subsequently injected into the well bore and selectively enters the formation at the interface of the two previously injected fluids.
U.S. Pat. No. 5,002,127 to Dalrymple et al selectively places a pair of treatment fluids in a formation by simultaneously, but separately, injecting the two fluids into the formation at different vertical locations determined by the positioning of a packer in the well bore. Commingling of the treatment fluids in the outlying formation is inhibited by injecting the two fluids at controlled injection pressures.
U.S. Pat. No. 3,369,605 to Donaldson et al discloses a process for selectively injecting a displacement gas into a formation to prevent water-coning. The gas is selectively directed into the formation by means of a packer positioned in the well bore and a series of sealed horizontal fractures formed in the near well bore region of the formation to drive hydrocarbons toward the well bore while driving water away from the well bore.
It is apparent from the above-described processes that an operationally simplified and more effective alternative is needed to selectively place a treatment material in a hydrocarbon production or fluid injection interval for inhibiting vertical flow between a well bore and an aquifer underlying the interval. Accordingly, it is an object of the present invention to satisfy these needs.
In particular, it is an object of the present invention to provide a process for selectively placing a treatment material in a near well bore region of a subterranean hydrocarbon-bearing formation, wherein the injection time of the treatment material is shortened to reduce the time that the well bore is inoperable. It is further an object of the present invention to provide a process for selectively placing a treatment material in a formation which avoids the use of complex, costly and often problematic mechanical packers. It is yet another object of the present invention to provide a process for selectively placing a treatment material in a formation which effectively avoids substantial entry of the treatment material into the upper portion of the interval being treated throughout the duration of the treatment and thereafter.